This invention relates to a glazed aluminum nitride (AlN) substrate suitable for use in applications such as thermal heads of a thermal recording device, thin film circuits, thick film circuits, and the like.
A glazed ceramic substrate, in which the unevenness of the ceramic substrate is eliminated by thinning and coating glass on the surface, is used, for example, in a thermal head of a thermal recording system. In such a thermal recording system, a thermal head is held in close contact with a thermally sensitive paper. By generating heat, a thermal resistor incorporated within the thermal head imprints a record on the thermally sensitive paper.
A glazed ceramic substrate is also widely used in thin-film circuit substrates. Because a high-frequency device is generally mounted on the substrate, a circuit substrate in a thin film hybrid IC must have a smooth surface. It must also have the properties of high precision, low noise, and the like. Such a glazed ceramic substrate may further be used in a thick-film circuit substrate to improve its properties.
By using as a substrate material a sintered AlN body having a greater thermal conductivity, the heat radiating property of the resultant glazed AlN substrate is improved. However, when the glazing material on an Al.sub.2 O.sub.3 substrate is a glass, such as an oxide glass of SiO.sub.2 --B.sub.2 O.sub.3 Al.sub.2 O.sub.3 --CaO, applied and formed by firing on the sintered AlN body, the glass layer breaks easily.
This fragility is due to the large difference in the coefficient of thermal expansion between the sintered AlN body and the glass layer. To prevent breakage of this nature, a glass having a relatively small coefficient of thermal expansion that approximates the coefficient of thermal expansion of the sintered AlN body has been proposed. However, using such a glass introduces an additional problem. A glass with a smaller coefficient of thermal expansion also has a higher softening point. Such a glass thus requires firing at elevated temperatures to secure the surface smoothness of the glass layer. Firing at elevated temperatures increases reactivity between the sintered AlN body and the glass layer. Bubbles generated by the resulting reaction produce unevenness, including blisters, on the surface of the glass layer, thereby adversely affecting the surface smoothness of the glass layer.
To prevent a reaction between the sintered AlN body and the glass layer, a glazed AlN substrate may be prepared by forming an SiO.sub.2 layer on the surface of the sintered AlN body through a surface oxidized layer. The glass layer adheres to the SiO.sub.2 layer, which prevents any undesirable reaction between the sintered AlN body and the glass layer.
The thickness of SiO.sub.2 layer is increased to correspond to that of the glass layer. As the thickness of the SiO.sub.2 layer is increased, there is an increased likelihood that shrinking cracks may occur in the SiO.sub.2 layer during the process of fire-forming. When these cracks occur, the glass component of the glass layer penetrates the sintered AlN body at the time the glass layer is fired at an elevated temperature. The result is a violent reaction, between the glass and the sintered AlN body, that produces bubbles in the glass layer. As a consequence, where a thicker glass layer is desired, it difficult to form it by firing at elevated temperatures without adversely affecting its surface smoothness.